The present invention relates to a document reading apparatus for imaging a document from above, and capable of detecting and rectifying curvatures of a document surface.
In case of imaging a document from above, an image of a document whose document surface is flat can be picked up exactly. However, in case of imaging a document bound in book form from above, a picked up image is distorted and compressed in regions adjacent to a joint, since document surface is curved along the joint. As a countermeasure to this inconvenience, it is conventionally known a document reading apparatus for imaging a document with use of a line sensor and the like, which employs a distance sensor for measuring a distance from the document and changes a reading pitch in the direction of sub scanning in accordance with measured value (e.g., Japanese Laid-Open Patent Publication No. 62-143557). Also, it is known an apparatus which illuminates a document surface with a line beam, measures a shape of the document, namely, how the document surface is curved, by making use of an image of the reflected light and rectifies the data by effecting an expansional processing with an image data in accordance with the measured shape (e.g., U.S. Pat. No. 5,084,611)
However, in the former apparatus, it is difficult to construct a mechanical arrangement so as to change a reading pitch in sub scanning direction, resulting in a complicated construction. Further, production costs has to become high because of the adoption of a distance sensor.
Meanwhile, in the latter apparatus, an interpolation of distance measurement data and the like is not carried out, and then it often occurs that a shape measurement is not performed precisely. Accordingly, it is impossible to rectify the image data accurately. Also, because a density reproducing interpolative processing is not performed at the time of expansional processing of a picked up image, it often occurs that an image is not reproduced clearly in the regions adjacent to a joint in a document whose image must be expanded at a large ratio. Also, it is impossible to rectify the distortion in the direction of line, caused by the displacement in the direction of row of the picked up image owing to the difference of height over the document surface.